We study competition among market designers who create new trading platforms, when boundedly rational traders learn to select among them. We ask whether efficient platforms, leading to market - clearing trading outcomes, will dominate the market in the long run. If several market designers are competing, we find that traders learn to select non-market clearing platforms with prices systematically above the market-clearing level, provided at least one such platform is introduced by a market designer. This in turn leads market designers to introduce non-market clearing platforms. Hence platform competition induces non-competitive market outcomes.